In order to suppress an on resistance while keeping a withstand voltage in a vertical power MOSFET that is a power semiconductor device, the use of a superjunction structure has been examined.
For example, Japanese Unexamined Patent Application Publication No. 2008-305927 discloses a semiconductor device having a superjunction structure where n-conductivity type columns and p-conductivity type columns are alternately arranged. Moreover, in this disclosure, an increase in the aspect of a PN column layer can further reduce the on resistance of a metal oxide semiconductor field-effect transistor (MOSFET) and increase the withstand voltage of the MOSFET. Furthermore, trenches tapered forward are formed on the top surface of an n-conductivity type epitaxial layer, and then a p-conductivity type column having an embedded epitaxial layer is formed in each of the trenches.